Visibly transparent dyes for through-transmission laser welding

ABSTRACT

Selection criteria for dyes that predicts efficiency and performance for plastics welding. A first stage quantitative calculation screens dyes that possess an absorption band that can be matched to a laser that is suitable for plastic welding. It also identifies absorption troughs in the visible spectrum and establishes relationships between the relative optical densities of the absorption band and trough. A second stage quantitative calculation screens dyes for their contribution to the transparency of the substrate. By combining these two stages, the usefulness of a candidate dye can be quickly, easily and inexpensively determined.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of co-pending U.S. patent application Ser. No.10/888,808 filed Jul. 9, 2004, which is a continuation of co-pendingU.S. patent application Ser. No. 10/439,776 filed May 19, 2003, now U.S.Pat. No. 6,911,262, which is a continuation of pending U.S. patentapplication Ser. No. 09/711,277 filed Nov. 10, 2000, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to visibly transparent dyes and a method for usingsame in through-transmission laser welding. More particularly, itrelates to the use of dyes in aesthetically demanding applications.

2. The Prior Art

Many different materials are used as brazing compounds or radiationscattering compounds in plastic welding. Generally the top substrate,through which the radiation is first transmitted before reaching thejoint region, is designed to transmit a high portion of the weldingradiation.

Brazing compounds are added to the joint region to absorb or scatter theincident radiation to melt the facing surfaces that comprise the jointto be welded. Radiation scattering is an uncontrolled and relativelyinefficient method of converting the incident radiation to heat.Accordingly, radiation scattering may require larger amounts of thebrazing compound, greater amounts of incident radiation, slower linearwelding speeds, and may result in a poor appearance of the final weld.

Other radiation scattering or radiation adsorbing materials may beincorporated into the surface of either substrate at the joint region,or into the entire lower substrate. However, there are no knowncompounds or welding techniques which can provide predicable results indemanding aesthetic industrial and consumer applications like automobilemoldings and food and beverage packaging. A brief description of thoseknown materials and methods follows:

U.S. Pat. No. 4,424,435 employs metal brazing compounds, preferablytitanium, but also silicon carbide, beryllium, cobalt, germanium, iron,molybdenum, nickel, niobium, platinum, rhenium, rhodium, tantalum andtungsten. Several severe limitations are inherent in the disclosedmethods. First, these metallic brazing compounds are only suitable forwelding glass. Second, the patent admits that some experiments withNd-YAG pulsed lasers have resulted in some nonlinear effects at the weldjoint which are not entirely understood.

U.S. Pat. No. 5,843,265 contemplates the use of inorganic materials,such as, but not limited to, pigments, fillers, fibers and reinforcingmaterials as radiation absorbing compounds. Their preferred embodimentemploys a carbon black suspension which necessarily tints the hostworkpiece to a muddy grey or black. In addition, the use of apolychromatic, non-coherent radiation source requires the presence ofcumbersome focusing elements and masks.

U.S. Pat. Nos. 4,156,626; 4,906,320; and 5,501,759 generically refer toprinter's ink, carbon black, and aniline dyes; opaque ink and othersufficiently opaque materials; and dark ink or dark polymeric film,respectively.

U.S. Pat. No. 5,893,959 refers to light scattering pigments or glassfibers. In order to hide the presence of the pigments, they prescribetinting both workpieces to be black and opaque.

The published PCT application bearing International Application NumberPCT/GB99/03241 discloses a host of organic dyes and metalated organicdyes such as cyanine dyes, squarylium dyes, croconium dyes, metalphthalocyanine dyes, metalated azo dyes and metalated indoaniline dyes.

The above survey indicates that there are a multitude of options forradiation absorbing compounds. An inordinate amount of experimentation,and expense associated with those rarer compounds, may be required toselect a general purpose dye for aesthetically demanding applications.Further confusing the issue are numerous interrelated factors such ashost selection and its polymerically related cousins, hostcompatibility, including solubility, the degree of visibility of thecompound, the type and degree of tinting contributed by the dye, ifvisible, and the related issues of host thickness, and dye concentrationas a function of host thickness, i.e. the concentration gradient.

SUMMARY OF THE INVENTION

It is an object of the invention to identify dyes having strongabsorption bands above 350 nm.

It is an object of the invention to identify dyes that further haveabsorption troughs in the visible region.

It is an object of the invention to identify dyes that are alsoextremely soluble, add little or no light scattering and efficientlyabsorb laser radiation and via vibronic relaxation transmit heat to weldplastics.

It is an object of the invention to additionally quantify the relativeoptical densities of the absorption band and absorption troughs.

It is an object of the invention to identify dyes that further stronglytransmit light across most or all of the visible spectrum.

It is an object of the invention to identify dyes that also have highphotopic transmission values.

It is an object of the invention to provide quantitative means foridentifying dyes that meet all of the above criteria.

These and other related objects are achieved according to the inventionby providing selection criteria for dyes that predict efficiency andperformance for plastics welding. A first stage quantitative calculationscreens dyes that possess an absorption band that can be matched to alaser that is suitable for plastic welding. It also identifiesabsorption troughs in the visible spectrum and established relationshipsbetween the relative optical densities of the absorption band andtrough.

A second stage quantitative calculation screens dyes for theircontribution to the transparency of the substrate. By taking intoconsideration the results from both stages, the usefulness of acandidate dye can be quickly, easily and inexpensively determined. Thekey is identifying the most critical parameters that determine a dyesability to perform in aesthetically demanding welding applications andsubsequently discovering ways to rate the dyes and compare them to eachother.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings to which reference is made in the instantspecification and which are to be read in conjunction therewith and inwhich like reference numerals are used to indicate like parts in thevarious views:

FIG. 1 is a graph showing the Absorbance Ratio points for thewavelengths of interest for the candidate dye of Example 1.

FIG. 2 is a graph showing the Absorbance Ratio points for the samewavelengths of interest for carbon black.

FIG. 3 is a graph showing the complete Absorbance Ratio curves for thecandidate dye compared to carbon black.

FIG. 4 is a graph showing the photopic curve combined with theilluminant C curve.

FIG. 5 is a graph combining the transmission curve of candidate dye fromExample 1 with the photopic and illuminant C curve of FIG. 4.

FIG. 6 is a graph combining the transmission curve of carbon black withthe photopic and illuminant C curve of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Through transmission welding by various sources of radiation is known.The use of lasers as the radiation source represents a subset of thiswelding technology. The arrangement of the laser oriented to passthrough an upper substrate to the joint is described in detail in U.S.Pat. No. 5,893,959, the contents of which are incorporated herein byreference thereto. Above and beyond the teachings of U.S. Pat. No.5,893,959, the present invention includes the features of: lasersmatched to absorption peaks above 350 nm, for example, Nd:YAG tripled(354), Nd:YAG doubled (532 nm), Argon (488 and 514 nm), Cu vapor (511and 578 nm), ruby (694.3 nm), HeNe (632.8 nm), Krypton (647 nm), VISdiode (about 600 to 780 nm) and dye lasers (577 to 593 nm); laserabsorbing dyes disposed in the upper substrate at the joint region;laser absorbing dyes disposed in a separate film or lamina clampedbetween the substrates; laser absorbing dyes disposed in the lowersubstrate at the joint region or thoughout all or part of the substrate;substrates selected from a variety of polymers, for example,polycarbonate, polymethylmethacrylate (PMMA), polyamides, and polyestersand films made of these polymers; polyolefin films; transparentsubstrates including clear, water-white and cosmetically tinted butstill transparent substrates; a variety of aesthetically demandingapplications; and novel Absorption Ratios to determine dye efficiencyand photopic values to determine dye performance.

The dye may be introduced into the joint region in several ways. First,the dye can be incorporated into a thin film. The film is preferablymade of the same material as at least one of the substrates. If the filmis made from a different material than a substrate, the film should becompatible with, i.e. soluble in, the polymer. The film may contain onthe order of one ten-thousandths part dye on a weight basis. The filmthickness may be on the order of tens of microns thick. Second, the dyemay be incorporated into a surface of either substrate facing the jointvia insert molding, dip coating, dye infusion, painting, printing,spraying. The dye may furthermore be incorporated into the entire lowersubstrate since only the surface facing the joint will be reactive.

Previously it was thought that laser welding wavelengths had to be atleast above 800 nm, outside the visible region, in order to be useful indemanding aesthetic applications. The theory was that any absorptionbands between 380 nm and 780 nm would strongly interfere with thetransmission of visible light resulting in darkly or intensely coloredportions by virtue of their dye content. Furthermore, it was thoughtthat incident laser radiation below 380 nm would adversely severpolymeric bonds.

Surprisingly it was discovered that only radiation below 350 nm wouldadversely affect the polymeric bonds. Even more astonishing was thediscovery that laser welding wavelengths in the visible spectrum couldbe used if the concentration of the dye was balanced against the dye'sabsorption bandwidth. Applicant has developed a novel two part ratingsystem to measure and balance dye efficiency and dye performanceespecially for through-transmission laser welding.

Part 1—Dye Efficiency. Dye concentration is a key issue that drives theneed for screening and selecting general-purpose dyes usable in a rangeof concentrations for different applications. Currently in order toassess the effect of different concentrations, a part needs to bereproduced at nominal thicknesses and doped at different levels so thatall the reproductions can be tested. This process may have to berepeated for parts of different thicknesses. The expense and requiredresources for such testing cycles may be commercially unreasonable.

In addition, it can be very difficult to test thin films or laminabecause reflections off the front and back surfaces can createinterference patterns that obfuscate the dye transmission data ordifferences between measured films. Furthermore, thicker pieces haveanother problem in that their own transmission characteristics may maskor overshadow what would otherwise be extremely high transmission bandsof the dye. For cosmetically tinted pieces, the transmissioncharacteristics of the cosmetic tint may further mask or overshadow thetransmission bands of the dye.

Applicants have developed a measure that they call “Absorbance Ratio”that measures the relative optical density (OD) as a function ofwavelength. The Beer Lambert Law, defines the OD at a given wavelength(wv) asOD(wv)=c·t·k(wv)where:

-   -   c is the molar concentration of the absorber in the host        material (mol/L);    -   t is the path length through the host material (cm); and    -   k is the molar extinction coefficient of the absorber in the        host material at the wavelength in question (L/(mol·cm))

Four separate optical density measurements are used to generate oneAbsorbance Ratio point at a given wavelength. This Absorbance Ratio usesthe dye's absorption band matched to a laser welding wavelength as areference point, having an optical density of unity, for example. Thisreference value is then compared to the absorbance troughs in thevisible spectrum. Both values subtract out the absorbance due to thesubstrate thereby making the ratio independent of substrate contributionor interference.

Applicants Absorbance Ratio may be calculated by preparing just twosubstrate samples of nominal thickness. Sample 1 will be devoid of thedye under investigation (Substrate) and Sample 2 will have the dyeincorporated therein at concentration X (Substrate+Dye).

Next, identify a laser wavelength which is within the dye's absorptionband. This will serve as the laser welding wavelength (lww) in thedenominator of all Absorbance Ratio equations for a given example. Then,plot optical density (OD) as a function of wavelength (wv) as follows inEquation 1: $\begin{matrix}{{{Absorbance}\quad{Ratio}\quad({OD})} = {\frac{{{OD}\left( {{Substrate} + {{Dye}@{wv}}} \right)} - {{OD}\left( {{Substrate}@{wv}} \right)}}{{{OD}\left( {{Substrate} + {{Dye}@{lww}}} \right)} - {{OD}\left( {{Substrate}@{lww}} \right)}}.}} & {{Equation}\quad 1}\end{matrix}$

EXAMPLE 1 Calculating the Absorbance Ratio for a Candidate Dye

The substrate was doped with a notch absorber at a concentration ofbetween 0.05 and 0.10 grams per pound of substrate. In this example, thenotch includes the 532 nm wavelength of the doubled Nd:YAG laser.Accordingly the laser welding wavelength equals 532 nm (lww=532 nm).This value used in the denominator of Equation 1 will be constant forall calculations with this absorber.

Next we calculate the reference point for our Absorbance Ratio. Thereference point by definition has an optical density (OD) of unity. Thereference point is where the wv equals lww (532 wl−532 lww) as iscalculated in Equation 1-A as follows: $\begin{matrix}{{{Absorbance}\quad{Ratio}\quad({OD})} = {\frac{{{OD}\begin{pmatrix}{{Substrate} +} \\{{{{Dye}@523}\quad{nm}}\quad}\end{pmatrix}} - \quad{{OD}\left( \quad{{{Substrate}@523}\quad{nm}} \right)}}{{{OD}\begin{pmatrix}{{Substrate} +} \\{{{{Dye}@523}\quad{nm}}\quad}\end{pmatrix}} - {{OD}\left( {{{Substrate}@523}\quad{nm}} \right)}} = 1}} & {{Equation}\quad 1\text{-}A}\end{matrix}$

Note that the Absorbance Ratio will remain at 1 independent ofconcentration, substrate type, substrate transmission characteristics orsubstrate thickness.

Next we calculate the Absorbance Ratio at a first visible wavelength. Asan example we have selected 592 nm, which is input into Equation 1-B asfollows: $\begin{matrix}{{{Absorbance}\quad{Ratio}\quad({OD})} = {\frac{{{OD}\begin{pmatrix}{{Substrate} +} \\{{{{Dye}@592}\quad{nm}}\quad}\end{pmatrix}} - \quad{{OD}\left( \quad{{{Substrate}@523}\quad{nm}} \right)}}{{{OD}\begin{pmatrix}{{Substrate} +} \\{{{{Dye}@592}\quad{nm}}\quad}\end{pmatrix}} - {{OD}\left( {{{Substrate}@523}\quad{nm}} \right)}} = {.01}}} & {{Equation}\quad 1\text{-}B}\end{matrix}$

Note that at this visible wavelength the Absorbance Ratio is yields anoptical density one-hundredth less dense than at the reference point.This is shown graphically as point B on FIG. 1. Optionally, we thencalculate the Absorbance Ratio at a second visible wavelength. As anexample we have selected 751 nm, which is input into Equation 1-C asfollows: $\begin{matrix}{{{Absorbance}\quad{Ratio}\quad({OD})} = {\frac{{{OD}\begin{pmatrix}{{Substrate} +} \\{{{{Dye}@751}\quad{nm}}\quad}\end{pmatrix}} - \quad{{OD}\left( \quad{{{Substrate}@751}\quad{nm}} \right)}}{{{OD}\begin{pmatrix}{{Substrate} +} \\{{{{Dye}@592}\quad{nm}}\quad}\end{pmatrix}} - {{OD}\left( {{{Substrate}@523}\quad{nm}} \right)}} = {.001}}} & {{Equation}\quad 1\text{-}C}\end{matrix}$

Note that at this visible wavelength the Absorbance Ratio yields asoptical density one-thousandth less dense than at the reference point.This is shown graphically as point C on FIG. 1.

To summarize, the lww A point serves as a reference point having anoptical density of 1. Applicants dye efficiency criteria is based on therelative value of point B. In the example, the B point is two orders ofmagnitude below the reference point. This suggests that the dyetransmits fairly well near the middle of the visible spectrum,represented by the B point, yet very efficiently absorbs at the lww. Asindicated by the C point, the dye also transmits extremely well at thehigh end of the visible spectrum.

While the example utilizes a laser welding wavelength in the visiblespectrum, it should be understood that other lasers outside the visiblewavelength may be selected. Applicants selection criteria is equallyvalid for near ultra-violet welding between about 350 nm and about 380nm, for example, with the Nd:YAG tripled at 354 nm. However, it iscritical that Equations 1-B, and optionally Equation 1-C, be calculatedat wavelengths within the visible spectrum.

EXAMPLE 2 Calculating the Absorbance Ratio for Carbon Black

Next we calculate the Absorbance Ratio for a prior art compound, forexample carbon black. We use the same reference point of 532 nm for thelww and we use the same wavelengths as above, 532 nm, 592 nm and 751 nm,for the wv value in the numerator. These points as shown as X, Y and Zon FIG. 2. The Absorbance Ratio at the reference point (X) is 1.0. TheAbsorbance Ratios at point Y is 1.0 and at point Z is 1.1. The followingtable presents a side-by-side comparison: Absorbance Ratios (ref. graphpoint) Wavelength 532 nm Absorber Carbon Black 532 nm 1.0 (A) 1.0 (X)592 nm 0.01 (B) 1.0 (Y) 751 nm 0.001 (C) 1.1 (Z)

FIG. 3 shows the full Absorbance Ratio curves for the 532 absorber andcarbon black together. This graph readily shows that the 532 absorbereffectively absorbs at the desired laser welding wavelength and absorbs100 times and 1000 times less strongly at other points within thevisible spectrum. Carbon black absorbs at about the same strength acrossthe entire visible spectrum. In general we will select dyes thatdemonstrate this 100-fold difference. The greater the difference, thebetter the dyes efficiency rating. However, it is to be noted that aselected dye according to the present invention may have an AbsorptionRatio at a wavelength within the dye's absorption trough that is lessthan one-tenth ( 1/10) of the dye's Absorbance Ratio at a wavelength inthe absorption band. This is part one of our selection criteria, parttwo is the photopic value.

Part 2—Dye Performance. The Photopic value is the eye-integrated valueof the filter over the visible spectrum as defined by Equation 2 asfollows: $\begin{matrix}{{{Photopic}\quad{Transmission}\quad\%} = {\frac{\int_{380\quad{nm}}^{780\quad{nm}}{{T(x)} \cdot {P(x)} \cdot {{IC}(x)} \cdot {\mathbb{d}x}}}{\int_{380\quad{nm}}^{780\quad{nm}}{{P(x)} \cdot {{IC}(x)} \cdot {\mathbb{d}x}}} \times 100\%}} & {{Equation}\quad 2}\end{matrix}$where:

-   -   T(x) represents the transmission values of the dye in question        as a function of wavelength.    -   P(x) represents the photopic sensitivity curve as a function of        wavelength.    -   IC(x) represents the reference light source (typically        illuminant C) to which the eye's photopic sensitivity is        calibrated as a function of wavelength.

FIG. 4 shows the P(x)·IC(x) curve. The area under the curve representsthe integral of P(x)·IC(x), i.e. the denominator of Equation 2.

FIG. 5 shows the transmission curve for the 532 nm dye overlying theP(x)·IC(x) curve from FIG. 4. The cross-hatched area under the combinedcurves, represents the numerator of Formula 2 above. In our presentexample, the area in FIG. 5 is about twenty percent less than the areaof FIG. 4. Accordingly, Equation 2 calculates an 80% photopic value forthe dye.

FIG. 6 shows the transmission curve for carbon black overlying theP(x)·IC(x) curve from FIG. 4. The cross-hatched area under the combinedcurves, represents the numerator of Formula 2 above. The area in FIG. 5is about forty percent less than the area of FIG. 4. Accordingly,Equation 2 calculates a 60% photopic value for carbon black.

For the sake of comparison, clear plastic, or more particularlywater-white polycarbonate possesses a photopic value of about 88-90%depending on thickness and other factors. Applicants have discoveredthat for a general purpose dye, a photopic value within 10% of waterwhite is ideal and will likely be unseen by the naked eye. Furthermore,a at slightly higher dye concentrations or dye concentration gradients,a photopic value within 15% of water white is highly desirable and mayprovide only the slightest hint of color under perfect viewingconditions. Finally, at greater dye concentrations or higher dyeconcentration gradients, a photopic value within 20% of water white ispractical and useful where a light pastel coloration to the lowersubstrate or joint region is acceptable.

Accordingly, we use high photopic values, within given ranges towater-white as the second performance criteria to evaluate generalpurpose through-transmission laser welding dyes.

DYE EXAMPLES

Below is an exemplary listing of three classes of dyes that achieved ahigh rating under the criteria of the invention. Other classes of dyeswith favorable ratings may also exist and are included under thisinvention.

A. The first class of dyes that has both high efficiency and highperformance under Applicants rating system is metalloporphyrins. Thisclass is characterized by cyclic planar compounds consisting of fourpyrrole rings bridged to each other by methyne carbon atoms and chelatedwith a metal ion bearing a +2 charge, such as Pt+2, Cu+2, or Zn+2. Thecandidate dye from Example 1 above is a palladium porphyrin.

B. The second class of dyes that has both high efficiency and highperformance under Applicants rating system is metalloazaporphyrins. Thisclass is characterized by cyclic planar compounds consisting of fourpyrrole rings bridged to each other by nitrogen atoms and chelated witha metal ion bearing a +2 charge, such as Pt+2, Cu+2, Zn+2, or Pd+2.

C. The third class of dyes that has both high efficiency and highperformance under Applicant's rating system is Fischer Base dyes. Thisclass is characterized by indolene molecules comprising a benzene ringfused to a pyrrole ring with the N in a position adjacent to thejuncture. A dienyl group is attached at the pyrrole carbon adjacent theN and is terminated with numerous molecular moieties which areconjugated with double bonded structures and with various alkylsubstituents on the indolene ring.

In general, candidate dyes would possess high extinction coefficients.Applicant's rating system then provides a quantitative way to measurewhether those candidate dyes also possess a sufficiently low AbsorbanceRatio within the visible spectrum as well as sufficiently high photopicvalues for the welding application in question.

As recited in the claims, a fraction means less than one-half. The wordtransparent means that images transmitted through a substrate areclearly discernable without linear or spatial distortion. Transparentincludes clear or water-white plastic panels. Transparent also includesplastic panels which are lightly pastel tinted but do not distort theshape or relationship of images, like lightly tinted portions ofautomobile windows.

1. A laser in combination with plastic substrates and an absorber dyefor through-transmission laser welding at a welding wavelengthcomprising: an upper plastic substrate being highly transmissive of thelaser wavelength and having a lower welding surface; a lower substratewith an upper welding surface; an interior joint region being formed byassembling the upper substrate on to the lower substrate with the lowerwelding surface facing the upper welding surface; an absorber dyedisposed in the interior joint region at a concentration and having (i)an absorption band above about 350 nm that includes the weldingwavelength, (ii) an absorption trough in the visible spectrum, and (iii)strong transmission of light in the visible spectrum and prior towelding said dye concentration providing a ratio comprising a substrateindependent optical density (OD) value within the (ii) absorption troughthat is a fraction of the substrate independent optical density (OD)value within the (i) absorption band; and a laser emitting radiationthrough said upper substrate to said interior joint region where saiddye efficiently absorbs the laser radiation to heat said weldingsurfaces to produce a weld in which the dye that is irradiated withinthe joint region remains disposed only within the weld and the matchingof the absorption band to the welding wavelength is independent ofplastic substrate contribution and interference.
 2. The combination ofclaim 1, wherein the strong transmission of light is across most of thevisible spectrum outside of the absorption band.
 3. The combination ofclaim 1, wherein the absorption band is outside the visible spectrumwhereby the strong transmission of light is across the entire visiblespectrum.
 4. The combination of claim 1, wherein the strong transmissionof light in the visible spectrum comprises a high photopic value for thedye.
 5. The combination of claim 4, wherein the photopic value of theplastic containing the dye is less than about 10% lower than thephotopic value of the plastic, whereby the presence of the dye withinthe plastic is invisible to the naked eye.
 6. The combination of claim4, wherein the photopic value of the plastic containing the dye iswithin about 10% to about 20% lower than the photopic value of theplastic, whereby the dye lends minimal coloration to the plastic andwherein the plastic containing the dye is transparent.
 7. Thecombination of claim 6, wherein the minimal coloration comprises a lightpastel coloration.
 8. The combination of claim 1, wherein the dye has anAbsorption Ratio of a wavelength within the absorption trough that is afraction of the dye's Absorbance Ratio of a wavelength in the absorptionband.
 9. The combination of claim 1, wherein the dye has an AbsorptionRatio of a wavelength within the absorption trough that is less thanone-tenth ( 1/10) of the dye's Absorbance Ratio of a wavelength in theabsorption band.
 10. The combination of claim 1, wherein the dye has anAbsorption Ratio of a wavelength within the absorption trough that isless than one-hundredth ( 1/100) of the dye's Absorbance Ratio of awavelength in the absorption band.
 11. The combination of claim 1,wherein the dye absorbs and transmits heat via vibronic relaxation. 12.The combination of claim 10, wherein the strong transmission of light inthe visible spectrum comprises a high photopic value for the dye. 13.The combination of claim 1, wherein the dye is incorporated into a thinfilm that is disposed between the lower welding surface and the upperwelding surface.
 14. The combination of claim 13, wherein the film ismade of a plastic that is the same as one or both of the substrates. 15.The combination of claim 13, wherein the film is on the order of tens ofmicrons thick.
 16. The combination of claim 13, wherein the filmcontains on the order of one ten-thousandths part absorber dye on aweight basis.
 17. The combination of claim 1, wherein the substrateindependent optical density (OD) value within the (ii) absorption troughcomprises OD(Substrate+Dye@wv)−OD(Substrate@wv).
 18. The combination ofclaim 1, wherein the substrate independent optical density (OD) valuewithin the (i) absorption band comprisesOD(Substrate+Dye@lww)−OD(Substrate@lww).
 19. The-combination of claim 1,wherein the dye has an Absorption Ratio of a wavelength within theabsorption trough that is less than one-thousandth ( 1/1000) of thedye's Absorbance Ratio of a wavelength in the absorption band.
 20. Thecombination of claim 1, wherein the dye has an Absorption Ratio of awavelength within the absorption trough that is a fraction of the dye'sAbsorbance Ratio of the welding wavelength in the absorption band. 21.The combination of claim 1, wherein the absorber dye is incorporated inone of the substrates, and the interior joint region comprises thewelding surface.
 22. The combination of claim 21, wherein the dye isincorporated in one of the welding surfaces by a process selected fromthe group consisting of insert molding, dip coating, dye infusion,painting, printing, and spraying.
 23. The combination of claim 22,wherein the dye is incorporated into the entire lower substrate.
 24. Thecombination of claim 1, wherein the dye has as Absorption Ratio thatyields optical density as a function of wavelength (wv) relative to areference wavelength, the Absorption Ratio is defined by the followingequation${{Absorbance}\quad{Ratio}\quad({OD})} = \frac{{{OD}\left( {{Substrate} + {{Dye}@{wv}}} \right)} - {{OD}\left( {{Substrate}@{wv}} \right)}}{{{OD}\left( {{Substrate} + {{Dye}@{lww}}} \right)} - {{OD}\left( {{Substrate}@{lww}} \right)}}$where lww is the reference wavelength.
 25. The combination of claim 24,wherein the reference wavelength is a wavelength above about 350 nm. 26.The combination of claim 24, wherein the reference wavelength is awavelength within the absorption band.
 27. The combination of claim 24,wherein the reference wavelength is the laser welding wavelength.
 28. Amethod of through-transmission laser welding with an absorber dye at alaser welding wavelength, comprising the steps of: providing an upperplastic substrate that is highly transmissive of the laser weldingwavelength and which has a lower welding surface, and providing a lowerplastic substrate that has an upper welding surface; assembling theupper substrate on to the lower substrate to form an interior jointregion that includes the lower welding surface and the facing upperwelding surface; incorporating an absorber dye into the interior jointregion at a concentration, wherein the absorber dye has (i) anabsorption band above about 350 nm that includes the welding wavelength,(ii) an absorption trough in the visible spectrum, and (iii) strongtransmission of light in the visible spectrum and prior to welding saiddye concentration providing a ratio comprising a substrate independentoptical density (OD) value within the (ii) absorption trough that is afraction of the substrate independent optical density (OD) value withinthe (i) absorption band; and emitting laser radiation through said uppersubstrate to said interior joint region where said dye efficientlyabsorbs the laser radiation to heat said welding surfaces to produce aweld in which the dye that is irradiated within the joint region remainsdisposed only within the weld and the matching of the absorption band tothe welding wavelength is independent of plastic substrate contributionand interference.
 29. The method of claim 28, wherein the strongtransmission of light is across most of the visible spectrum outside ofthe absorption band.
 30. The method of claim 28, wherein the absorptionband is outside the visible spectrum whereby the strong transmission oflight is across the entire visible spectrum.
 31. The method of claim 28,wherein the strong transmission of light in the visible spectrumcomprises a high photopic value for the dye.
 32. The method of claim 31,wherein the photopic value of the plastic containing the dye is lessthan about 10% lower than the photopic value of the plastic, whereby thepresence of the dye within the plastic is invisible to the naked eye.33. The method of claim 31, wherein the photopic value of the plasticcontaining the dye is within about 10% to about 20% lower than thephotopic value of the plastic, whereby the dye lends minimal colorationto the plastic and wherein the plastic containing the dye istransparent.
 34. The method of claim 33, wherein the minimal colorationcomprises a light pastel coloration.
 35. The method of claim 28, whereinthe dye has an Absorption Ratio of a wavelength within the absorptiontrough that is a fraction of the dye's Absorbance Ratio of a wavelengthin the absorption band.
 36. The method of claim 28, wherein the dye hasan Absorption Ratio of a wavelength within the absorption trough that isless than one-tenth ( 1/10) of the dye's Absorbance Ratio of awavelength in the absorption band.
 37. The method of claim 28, whereinthe dye has an Absorption Ratio of a wavelength within the absorptiontrough that is less than one-hundredth ( 1/100) of the dye's AbsorbanceRatio of a wavelength in the absorption band.
 38. The method of claim28, wherein the dye absorbs and transmits heat via vibronic relaxation.39. The method of claim 37, wherein the strong transmission of light inthe visible spectrum comprises a high photopic value for the dye. 40.The method of claim 28, wherein the dye is incorporated into a thin filmthat is disposed between the lower welding surface and the upper weldingsurface.
 41. The method of claim 40, wherein the film is made of aplastic that is the same as one or both of the substrates.
 42. Themethod of claim 40, wherein the film is on the order of tens of micronsthick.
 43. The method of claim 40, wherein the film contains on theorder of one ten-thousandths part absorber dye on a weight basis. 44.The method of claim 28, wherein the substrate independent opticaldensity (OD) value within the (ii) absorption trough comprisesOD(Substrate+Dye@wv)−OD(Substrate@wv).
 45. The method of claim 28,wherein the substrate independent optical density (OD) value within the(i) absorption band comprises OD(Substrate+Dye@lww)−OD(Substrate@lww).46. The method of claim 28, wherein the dye has an Absorption Ratio of awavelength within the absorption trough that is less than one-thousandth( 1/1000) of the dye's Absorbance Ratio of a wavelength in theabsorption band.
 47. The method of claim 28, wherein the dye has anAbsorption Ratio of a wavelength within the absorption trough that is afraction of the dye's Absorbance Ratio of the welding wavelength in theabsorption band.
 48. The method of claim 28, wherein the absorber dye iscombined with one of the substrates prior to said assembling step. 49.The method of claim 48, wherein the dye is incorporated in one of thewelding surfaces by a process selected from the group consisting ofinsert molding, dip coating, dye infusion, painting, printing, andspraying.
 50. The method of claim 49, wherein the dye is incorporatedinto the entire lower substrate.
 51. The method of claim 28, wherein thedye has as Absorption Ratio that yields optical density as a function ofwavelength (wv) relative to a reference wavelength, the Absorption Ratiois defined by the following equation${{Absorbance}\quad{Ratio}\quad({OD})} = \frac{{{OD}\left( {{Substrate} + {{Dye}@{wv}}} \right)} - {{OD}\left( {{Substrate}@{wv}} \right)}}{{{OD}\left( {{Substrate} + {{Dye}@{lww}}} \right)} - {{OD}\left( {{Substrate}@{lww}} \right)}}$where lww is the reference wavelength.
 52. The method of claim 51,wherein the reference wavelength is a wavelength above about 350 nm. 53.The method of claim 51, wherein the reference wavelength is a wavelengthwithin the absorption band.
 54. The method of claim 51, wherein thereference wavelength is the laser welding wavelength.